1. Field of the Invention
This invention relates to pneumatic tires having a unidirectional tread pattern, and more particularly to a pneumatic tire having improved cornering stability and noise reduction on off road or snow road conditions by improving the tread pattern of the tire.
2. Related Art Statement
Since the pattern of grooves formed in the tire tread has a large influence upon steering, braking, noise generation and the like of automotive vehicles, many kinds of patterns are designed from old concepts. Especially, tires having a pattern with a combination of grooves extending in the circumferential direction of the tire and slant grooves obliquely extending from the tread end toward the equator of the tire are used as a tire for off road or snow road conditions because of a high gripping force against road surface.
FIG. 6 is a plan view of a tread pattern of the conventional tire provided with slant grooves, wherein T is a tread, Gc is an equator of the tire, Ta and Tb are both tread ends, TW is a tread width and R is a rotational direction of the tire. In the tread pattern of FIG. 6, slant grooves 1a and 1b obliquely extend from both tread ends toward the equator of the tire. On the other hand, circumferential grooves 2a, 3a or 2b, 3b extending in parallel to the equatorial plane of the tire and continuously in the circumferential direction thereof cross over the slant groove 1a or 1a to form X-shaped crossroads.
As shown in FIG. 6, when the X-shaped crossroad is formed by crossing the circumferential groove over the slant groove, the road gripping force in the edge portion of each block on the tire tread, i.e. the edge effect thereof lowers on the off road or snow road due to the presence of X-shaped crossroads as the groove. The tire having this tread pattern is liable to interfere with the running because it is difficult to increase a component perpendicular to the equator of the tire, i.e. an axial component from a viewpoint of the pattern structure. In other words, tires having such a pattern are effective for the reduction of noise, but have a problem on the running performances.
As mentioned above, the tread pattern of the tire for the automotive vehicles is recently demanded to simultaneously possess the excellent running performances and the prevention of noise generation.